Foil bearings are used in a multiplicity of applications, particularly in high-speed applications, for example in thermal flow machines, turbochargers, compressors, etc.
Foil bearings are a special type of plain bearing. They are aerodynamic bearings which are designed for high rotational speed. Foil bearings of the first generation consist of what is referred to as the top foil, the supporting foil and the bearing back. In order to improve the stability of a shaft in said bearing back, a plurality of, typically three, seal shim rings (what are referred to as “shims”) distributed over the circumference are placed between the bearing back and the supporting foil. Without such seal shim rings, the aerodynamic buildup of pressure only takes place at a point in the circumferential direction of the shaft, namely where the shaft is caused by an external force to lie against the top foil.
FIG. 4 shows a schematic view for explaining an example of a foil bearing 400 without seal shim rings.
The foil bearing 400 has a bearing back 101, a shaft 103, a foil arrangement 107 with a top foil 107b and a supporting foil 107a. The foil arrangement 107 is arranged in a gap 403 between the shaft 103 and the bearing back 101. In FIG. 3, the shaft 103 lies against the top foil 107b only at one cross-sectional point. This gives rise to a gap 403 which narrows in the circumferential direction and in which the hydrodynamic or aerodynamic pressure can build up. The distribution of pressure in the foil bearing is sketched schematically with the curve 401.
FIG. 5 illustrates a further example of a foil bearing 500 with seal shim rings 501, 503, 505.
The foil bearing 500 has a bearing back 101, a shaft 103, a foil arrangement 507, 507′, 507″ each having a top foil 507b and a supporting foil 507a. The foil arrangement 107 is arranged in a gap 509 between the shaft 103 and the bearing back 101. In order to stabilize the shaft, the seal shim rings 501, 503, 505 are additionally arranged in the gap.
It can be seen in FIG. 5 that, by means of the seal shim rings 501, 503, 505, narrowing gaps arise at a plurality of points over the bearing circumference between the shaft 103 and the top foil 507b. At each of said narrowing portion, an aerodynamic pressure builds up in each case, said aerodynamic pressure supporting the shaft and positioning the latter in the radial direction. The position and the height of the seal shim rings 501, 503, 505 define the geometry of the construction. However, after the foil arrangement 507, 507′, 507″ and the seal shim rings 501, 503, 505 are installed, the geometry of the bearing 500 can no longer be changed, and the position and the strength of the pressure gradients are then only still dependent on the operating conditions (speed, viscosity of the medium and the temperature).
US 2004/0042691 A1 discloses a foil bearing with a plurality of foils which are in the shape of segments of a cylinder and are arranged in a gap between a shaft and a bearing back. The bearing back here is pierced by pins, wherein the length by which the pins project out of the bearing back can be set by a rotatable ring surrounding the bearing back.
US 2003/0118257 A1 shows a foil bearing with a multiplicity of piezoelectric actuators. The foil bearing comprises a housing and also a plurality of foils for supporting a rotating shaft. The piezoelectric actuators are designed to set the stiffness and the damping coefficient of the bearing.